qemu-arm: Add persistent environment support

qemu-arm: Add persistent environment support

Currently on qemu-arm platforms environment is kept in RAM. Instead
use pflash device 1 to provide persistent environment support across
device reset.

Also (optionally) provide support for persistent environment across
qemu machine OFF/ON using following instructions:

- Create envstore.img using qemu-img:
    qemu-img create -f raw envstore.img 64M
- Add a pflash drive parameter to the command line:
    -drive if=pflash,format=raw,index=1,file=envstore.img

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>